Transparent zstd compression for cache blobs and configurable symlink capture

Cache blobs are now transparently compressed with zstd when the result is smaller than the original, while the BLAKE3 digest stays based on the uncompressed bytes. This preserves cache compatibility with existing uncompressed blobs and reduces on-disk and remote cache usage for large, compressible artifacts. The feature is accompanied by a new output-symlinks annotation option that controls how symlinked outputs are captured.